Electrolytic phosphating process

ABSTRACT

This invention provides a phosphating process that can form a film suitable for a cold forging foundation within 60 seconds and preferably 30 seconds or less. The process uses a treatment bath that is formed of a phosphate ion solution (H 2 PO 4   − +Zn 2+ ), made by dissolving zinc in phosphoric acid, contains phosphoric acid (H 3 PO 4 ), phosphate ions, zinc ions and nitrate ions, may contain at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions, and further contains 0.5 g/l or below of metal ions other than the film forming components. The process involves electrolytic treatment by applying a voltage between a metal as a positive electrode and a treated article as a negative electrode and forms a phosphate film on the surface of the treated article. The phosphate ion solution prepared by dissolving zinc in phosphoric acid (H 2 PO 4   − +Zn 2+ ) is a solution obtained by dissolving 8 parts by mass to a maximum dissolution concentration of zinc in 100 parts by mass of phosphoric acid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrolytic phosphating process for forming a phosphate film, mainly composed of zinc phosphate, on a surface of a treated article. More specifically, the invention relates to a process for forming a film on a surface of a metal material having electric conductivity.

2. Description of the Related Art

A phosphating process is a treatment technology that is utilized for coating a foundation treatment of steel materials, a cold-forging lubrication treatment, and so forth. The phosphating process that is predominant at present is a non-electrolytic process but a variety of electrolytic phosphating processes have been examined in order to conduct a more efficient phosphating process.

Japanese Unexamined Patent Publication (Kokai) No. 2000-234200, for example, describes a prior art of such an electrolytic phosphating process and describes a basic method of using electrodes and a power source when the electrolytic phosphating process is executed. The reference mentions the dissociation state of acids in relation to a treatment bath and describes that the bath does not contain acids having a greater dissociation (nitric acid, for example) than phosphoric acid.

Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2002-322593 describes measures necessary for continuously carrying out the electrolytic phosphating process and describes also that intermediate products such as a N₂O₄ gas formed during the intermediate reaction process must be removed. However, these prior art technologies require a treatment time of 2 minutes or more and have not acquired methods for further shortening the electrolytic phosphating process for processed articles which have complicated shapes.

On the other hand, an electrolytic process has been employed, in the past, for steel wires and shortening of the process time has also been studied. Japanese Unexamined Patent Publication (Kokai) No. 2000-80497 discloses a novel method, and a novel apparatus, for forming a phosphate film having a higher performance as a lubrication foundation, on steel wires of low carbon steel, high carbon steel and low alloy steel, more quickly than the prior art and without generating any sludge. In this reference, the wire material shown in FIG. 1 is sequentially processed in each step. This mode of processing is peculiar to a wire material. The a material has a constant rod-like shape and its shape is advantageous for surface-electrolyzation. The object of the lubrication treatment (phosphate film) is drawing of the steel wire material (processed from large wire diameter to small wire diameter) and is limited plastic working. This object is basically different from the object of the present invention, that is, cold forging of the treated articles as components having complicated shapes including convex and concave shapes and requiring high machining precision, and cannot be applied to such articles.

It can be appreciated, on the other hand, that Japanese Patent No. 3,479,609 is directed to preventing sludge and to speeding-up the treatment, and presumably can be applied to the machining of components because the Examples use test panels and not wire materials. However, because nitric acid (HNO₃) is added to the treatment bath in this reference, the treatment bath is a bath that uses acids having a higher degree of dissociation than phosphoric acid. The reference further describes that the cover ratio of the film is 50% and this is not sufficient when a cathodic electrolytic process is carried out for 10 seconds in a treating solution composed only of phosphoric acid, nitric acid and zinc carbonate. Sodium nitrite, etc, is added to the bath so as to achieve a 100% cover ratio. In other words, this reference requires an additive containing soluble ions that do not precipitate as the phosphate film in order to promote film formation.

SUMMARY OF THE INVENTION

Because an electrolytic phosphating process supplies electrolytic energy (current/voltage) by directly connecting a treated article to an external D.C. power source, the efficiency of the electrolytic reaction is more satisfactory than the non-electrolytic process of the prior art. Therefore, this process can shorten the treatment time (electrolytic time) in comparison with the non-electrolytic system of the prior art.

The time required for the phosphating process is generally 120 seconds for processing the coating foundation and 5 to 10 minutes for processing the foundation for cold forging in the non-electrolytic system according to the prior art.

The cold forging foundation requires a larger film thickness than the coating foundation and the treatment time becomes longer, too. Therefore, cold forging foundation requires shortening of the treatment time. An object of the invention is to provide a phosphating process capable of forming a film suitable for cold forging within a short time of 60 seconds or less and preferably 30 seconds or less. When the treatment time can be shortened, the size of treatment equipment can be reduced and a dedicated machine can be constituted. Such a reduction is effective for a smaller installation area, a transfer line construction, reduction of intermediate stocks, saving man-power and energy, and so forth, and these are the objects to be accomplished in practical utilization of component machining.

A system for forming a film containing a phosphate by electrolysis from a treatment bath (solution) is described in Japanese Unexamined Patent Publication (Kokai) No. 2000-234200 described above. The present invention uses the system of this reference as the basis and forms a film mainly composed of zinc phosphate (crystal) by further developing this system.

To accomplish the object described above, the present invention provides the following inventions.

(1) An electrolytic phosphating process using a treatment bath that is formed of a phosphate ion solution made by dissolving zinc in phosphoric acid, contains phosphoric acid, phosphate ions, zinc ions and nitrate ions, may contain at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions and, further, contains 0.5 g/l or less of metal ions other than the film forming components, the method comprising the step of conducting an electrolytic treatment by applying a voltage between the metal as a positive electrode and a treated article as a negative electrode and forming a phosphate film on the surface of the treated article.

(2) The electrolytic phosphating process as described in (1), wherein the electrolytic treatment is carried out by using a treatment bath the pH of which is adjusted to 1.5 to 2.5 and the ORP (oxidation-reduction potential) of which is kept at 90 to 450 mV (silver/silver chloride electrode potential).

(3) The electrolytic phosphating process as described in (1) or (2), in which the bath contains at least 15 g/l of phosphoric acid and phosphate ions, at least 15 g/l of zinc ions, at least 12.5 g/l of nitrate ions, and 0 to 3 g/l of at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions.

(4) The electrolytic phosphating process as described in any of (1) through (3), wherein the positive electrode is selected from zinc or iron.

(5) The electrolytic phosphating process as described in any of (1) through (4), wherein, after anodic electrolysis is carried out by using the treated article as the positive electrode and zinc or iron as the negative electrode, cathodic electrolysis is carried out by using the treated article as the negative electrode and zinc or iron as the positive electrode.

(6) The electrolytic phosphating process as described in any of (1) through (5), wherein a voltage of not greater than 6 V is applied through connection to a D.C. power source.

(7) The electrolytic phosphating process as described in any of (1) through (6), wherein the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺), made by dissolving zinc in a phosphate ion solution, is a solution prepared by dissolving 8 parts by mass to a maximum dissolution concentration of zinc in 100 parts by mass of phosphoric acid.

(8) The electrolytic phosphating process as described in any of (1) through (7), wherein the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺), made by dissolving zinc in a phosphate ion solution, is a solution prepared by dissolving 15 to 25 parts by mass of zinc in 100 parts by mass of phosphoric acid.

(9) The electrolytic phosphating process as described in any of (1) through (8), wherein the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺) dissolving zinc in a phosphate ion solution is a solution prepared by dissolving zinc oxide, zinc hydroxide or metallic zinc in a phosphate ion solution.

(10) An electrolytic phosphating process as described in any of (1) through (9), wherein a ratio of the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺) and phosphoric acid (H₃PO₄) in the electrolytic treatment bath has a ratio of 0.4 to 1 represented by the relation [phosphate ion solution in which zinc is dissolved (H₂PO₄ ⁻+Zn²⁺)]/[phosphate ion solution in which zinc is dissolved (H₂PO₄ ⁻+Zn²⁺)+phosphoric acid (H₃PO₄)].

(11) The electrolytic phosphating process as described in any of (1) through (10), wherein the electrolytic treatment bath contains the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺) dissolving zinc in a phosphate ion solution, phosphoric acid (H₃PO₄) and zinc nitrate, and may contain a metal nitrate constituted by at least one kind of nitrate selected from the group consisting of nickel nitrate, cobalt nitrate, copper nitrate and manganese nitrate.

(12) The electrolytic phosphating process as described in any of (1) through (11), wherein the electrolytic treatment is carried out at a cathodic electrolysis current density of 1 to 18 A/dm² to form a phosphate film on the surface of the treated article.

(13) The electrolytic phosphating process as described in any of (1) through (12), wherein the electrolytic treatment is carried out under conditions free of any obstacles that may impede the flow of a current between the positive electrode and the negative electrode to form a phosphate film on the surface of the treated article.

(14) The electrolytic phosphating process as described in any of (1) through (13), wherein a zinc phosphate film is formed by setting the electrolytic treatment time to 60 seconds or below.

(15) The electrolytic phosphating process as defined in any of (1) through (14), wherein two or more kinds of voltages and currents are applied to one treated article depending on the position thereof by using two or more power sources and electrodes.

The present invention can shorten the phosphating treatment time of components as treated articles. Shortening of the treatment time allows a small installation area and in-line (transfer line) construction. In other words, because a large phosphating treatment apparatus according to the prior art is installed independently of previous and subsequent process steps, components are stored before and after the treatment apparatus. However, because the size can be reduced, the phosphating treatment apparatus can be installed adjacent to the previous and subsequent installation steps. In consequence, storage of the components before and after the phosphating treatment apparatus can be eliminated and man-power required for transportation and storage can be omitted.

The present invention allows a decrease in the size of the installation and lowering of the temperature and thus reduces the use amount of energy for heating, etc. Because the invention does not invite unnecessary reactions, the invention can prevent the formation of by-products (sludge) and reduces the amount of chemicals used.

The effect of the invention contributes to an improvement in the resulting film. This is obvious from the comparison of the films of later-appearing Examples 7 and 8 with the film of later-appearing Comparative Example 4. In Examples 7 and 8, fine zinc phosphate crystals are formed uniformly on the surface but a film of large crystals are coarsely formed in Comparative Example 4. This difference in the surface conditions reflects the difference of the deposition amounts of lubrication effective components. In other words, a uniform zinc phosphate crystal is advantageous for the formation of the lubrication effective components and is hence effective for cold forgeability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a basic phosphating process system according to the present invention;

FIG. 2 shows the formation state of films in the Examples and the Comparative Examples;

FIG. 3 shows the formation state of films in the Examples and the Comparative Examples;

FIG. 4 shows a current that flows when a 3 V voltage is applied between a zinc electrode and a test piece in cathodic electrolysis;

FIG. 5 shows a shape of a component used in Examples 7 and 8 and in Comparative Example 4, wherein formation processing is to change the shape from the one shown in FIG. 5 to a structure shown in FIG. 6;

FIG. 6 shows a gear-like structure of the component shown in FIG. 5 after cold forging press;

FIG. 7 is an appearance view of a phosphate film formed in Example 7;

FIG. 8 is an appearance view of a phosphate film formed in Example 8;

FIG. 9 is an appearance view of a phosphate film formed in Comparative Example 4; and

FIG. 10 shows a component B (tubular component having a cylindrical hollow portion) used for the phosphating process in Example 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an electrolytic phosphating process according to the invention, a treatment bath is prepared by using a phosphate ion solution (H₂PO₄ ⁻+Zn₂ ⁺) made by dissolving zinc in phosphoric acid. This solution contains phosphoric acid (H₃PO₄) and phosphate ions, zinc ions and nitrate ions, and may further contain at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions. The content of metal ions other than the film forming components are not greater than 0.5 g/l. The phosphate film is formed on the surface of the treated article by conducting electrolytic treatment by using a metal as the positive electrode and the treated article as the negative electrode and applying a voltage.

For example, the electrolytic treatment bath according to the invention appropriately contains the phosphate ion solution dissolving zinc in phosphoric acid (H₂PO₄ ⁻+Zn₂ ⁺), phosphoric acid (H₃PO₄) and zinc nitrate, and metal nitrates constituted by at least one kind of metal selected from nickel, cobalt, copper and manganese. As for iron, iron is preferably supplied from the solution made by dissolving iron in phosphoric acid in the same way as zinc. Iron nitrate is not desirable because it exists in the form of ferric salts (Fe³⁺), (Fe(NO₃)₃), and forms sludge.

Sodium and potassium are examples of metal ions other than the film forming components and the treatment bath according to the invention does not substantially contain these metal ions (not greater than 0.5 g/l).

The main constituent elements that constitute the electrolytic phosphating process system are “treatment bath” and “electrolytic method”. The electrolytic phosphating process according to the invention is accomplished by examining in detail the “treatment bath” and the “electrolytic method” in order to shorten the film formation time in the electrolytic phosphating process.

i) Examination of “Treatment Bath”-1:

Control of Dissociation Conditions of Phosphoric Acid

Precipitation of zinc phosphate is a phenomenon (reaction) in which phosphoric acid enters the dissociation state as represented by H₃PO₄→H₂PO₄ ⁻→PO₄ ³⁻, couples with zinc ions (Zn²⁺) and forms zinc phosphate (Zn₃(PO₄)₂). In this reaction process H₃PO₄→H₂PO₄ ⁻→PO₄ ³⁻, acidity becomes higher towards the left side in the solution state but the phosphate compounds (crystal:solid) is formed towards the right side, or the solution becomes alkaline. The formation of zinc phosphate in the film form by the electrolytic process is to electrolyze the solution containing phosphoric acid, phosphate ions and zinc ions in the solution state, to promote dissociation of phosphoric acid (namely, as represented by H₃PO₄→H₂PO₄ ⁻→PO₄ ³⁻), and to cause the zinc phosphate crystal (Zn₃(PO₄)₂) to precipitate as the film on the surface of the treated article. Therefore, the phosphating treatment bath must be in the solution state and dissociation of phosphoric acid must be limited to the range of H₃PO₄→H₂PO₄ ⁻.

The electrolytic treatment bath is preferably transparent. To facilitate precipitation of the phosphate, the dissociation state of phosphoric acid must be promoted in the treatment bath. The dissociation state of phosphoric acid can be promoted by dissolving zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂) or metallic zinc (Zn) in the H₃PO₄ solution. In other words, dissociation of phosphoric acid can be promoted and controlled by the following reaction formulas. 2H₃PO₄+ZnO→2H₂PO₄ ⁻+Zn²⁺+2H₂O  (1) 2H₃PO₄+Zn(OH)₂→2H₂PO₄ ⁻+Zn²⁺+2H₂O  (2) 2H₃PO₄+Zn→2H₂PO₄ ⁻+Zn²⁺+H₂  (3)

The reaction formulas (1) to (3) described above can be carried out substantially stably in a solution dissolving from 8 parts by mass to a maximum dissolution concentration of Zn²⁺ in 100 parts by mass of H₃PO₄ in a weight ratio. Preferably, the phosphate ion solution dissolving zinc contains 15 to 25 parts by mass of zinc in 100 parts by mass of phosphoric acid. In the present invention, the solution dissolving from 8 parts by mass to a maximum dissolution concentration of Zn²⁺ in 100 parts by mass of H₃PO₄ may be called the “element component consisting of H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight” for the convenience sake. In this way, the element component composed only of “H₃PO₄” and the element component composed of “H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight” can be used in the invention. The component “H₃PO₄” is the constituent component of the prior art but “H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight” is the constituent component that is proposed by the present invention. In other words, it is clear that the dissociation state of the phosphoric acid ion is different between pure “H₃PO₄” and “H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight”.

As described above, the invention uses two kinds of components, that is, [H₃PO₄] and [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] as the phosphoric acid components constituting the phosphating treatment bath. The invention proposes a ratio [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight]/[H₃PO₄]+[H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] (weight ratio) and defines it as [zinc dissolving phosphate ion solution ratio] (also called [Zn 25% dissolving phosphate ion solution ratio]).

Therefore, the solution of [zinc dissolving phosphate ion solution ratio]=1 means that phosphoric acid constituting the treatment bath wholly consists of [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight]. The solution of [zinc dissolving phosphate ion solution ratio]=0.5 means that 50% of phosphoric acid constituting the treatment bath is [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] and the rest consists of pure [H₃PO₄]. The solution of [zinc dissolving phosphate ion solution ratio]=0 means that phosphoric acid constituting the treatment bath consists of only pure [H₃PO₄].

The zinc dissolving phosphate ion solution ratio in the electrolytic treatment bath according to the invention, that is, [phosphate ion solution dissolving zinc in phosphate ion solution (H₂PO₄ ⁻+Zn²⁺)]/[phosphate ion solution dissolving zinc in phosphate ion solution (H₂PO₄ ⁻+Zn²⁺)+phosphoric acid (H₃PO₄)] is suitably from 0.4 to 1.

A treatment bath having a small [zinc dissolving phosphate ion solution ratio] is a treatment bath having a large pure [H₃PO₄] ratio and large (dissociation) energy is necessary in the solution to promote dehydrogenation of H₃PO₄. In contrast, a treatment bath having a large [zinc dissolving phosphate ion solution ratio] is a treatment bath having a large [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] ratio and dehydrogenation of H₃PO₄ can be promoted more easily than in the former. Therefore, the latter solution can precipitate the zinc phosphate crystal more easily and with a smaller electrolytic energy than the former.

By the way, the dissociation condition of phosphoric acid has also a correlation with the hydrogen ion concentration (pH) of the treatment bath and it is most suitable to carry out the electrolytic treatment by using a treatment bath that is kept at a pH of 1.5 to 2.5.

ii) Examination of “Treatment Bath”-2:

Observation of Soluble Metal Ions (Iron Ions) in Phosphoric Acid Other Than Zinc

Iron is a metal that is soluble in the phosphate ion solution and exhibits a similar behavior to zinc, in phosphoric acid. Iron is often the treated material, and the possibility of dissolution from the treated material into the treatment bath exists. Therefore, the behavior of iron ions must be understood.

Iron dissolves in phosphoric acid only from the metallic state. The condition is as follows. 2H₃PO₄+Fe→2H₂PO₄ ⁻+Fe²⁺+H₂  (4)

In the formula (4), Fe can be dissolved substantially up to Fe²⁺:20 mass parts in 100 parts by mass of H₃PO₄ in terms of the mass ratio.

Unlike zinc, Fe cannot be dissolved basically from the state of the oxide and hydroxide in phosphoric acid. Therefore, dissolution of the oxide and hydroxide of iron is affected by: Fe²⁺→Fe³⁺ +e:0.77V  (5)

The formula (5) represents also that Fe²⁺ is oxidized to Fe³⁺. This oxidation is generally promoted by oxygen in normal air. Under such a normal condition, iron oxide (FeO) and iron hydroxide (Fe(OH)₂) are affected and oxidized by oxygen, and exist in the forms of Fe₂O₃ and Fe(OH)₃, respectively. Such iron oxides cannot dissolve in phosphoric acid.

The relation of the formula (5) also affects the stability of the phosphating treatment bath. In other words, the Fe ions have the tendency to change from Fe²⁺→Fe³⁺ in the phosphating treatment bath. However, because solubility of Fe³⁺ is far smaller than that of Fe²⁺, large amounts of sludge are formed in the treatment bath with this change (Fe²⁺→Fe³⁺). Such a phenomenon is by no means desirable in the phosphating treatment bath.

Therefore, in the present invention that is directed to form a film containing zinc phosphate as the main constituent, the Fe ions that can be contained in the phosphating treatment bath are supplied from the solution dissolving in phosphoric acid and the dissolution amount (concentration) is preferably limited. It is about 3 g/l. When the concentration is higher, large amounts of sludge are undesirably formed in the treatment bath according to the method of the invention.

As for metals other than Fe, Mn is, for example, Mn²⁺:0.5 parts by weight on the basis of H₃PO₄:100 parts by weight in terms of a maximum weight ratio. It is difficult to form an Mn phosphate film at this concentration. In the present invention, 0 to 2 g/l of metal ions of other metals that do not dissolve in the phosphate ion solution, such as nickel, cobalt or copper, may be contained.

iii) Examination of [Electrolytic Method]-1

Restriction and Control of Reactions Other Than Phosphoric Acid Dissociation

The formation of the film by dissociation of phosphoric acid is carried out by limiting the condition of phosphoric acid of the treatment bath up to the range of [H₃PO₄→H₂PO₄ ⁻], and promoting [H₃PO₄→H₂PO₄ ⁻]→PO₄ ³⁻ and dissociation by electrolysis to form the film on the surface of articles. In other words, PO₄ ³⁻ and zinc ions (Zn²⁺) are coupled on the surface of the negative electrode (treated article) and the zinc phosphate (Zn₃(PO₄)₂) crystal is formed as the film.

It is of the utmost importance that other reactions (reaction such as electrolysis of water) are suppressed. Reaction products other than phosphate are formed when the reactions other than dissociation of phosphoric acid occur (with the proviso that a controlled reaction is permitted, whenever necessary).

Dissociation of phosphoric acid can be carried out at a voltage lower than the voltage for the electrolysis of water. More specifically, dissociation of phosphoric acid is promoted but electrolysis is restricted to an impressed voltage of 6 V or below (refer to Japanese Unexamined Patent Publication (Kokai) No. 2004-52058, for details). The invention particularly employs the treatment bath that is prepared by using the phosphate ion solution dissolving zinc in phosphoric acid and promotes in advance dissociation of phosphoric acid. Therefore, the invention can easily dissociate phosphoric acid at a voltage of 6 V or below until the coating can be precipitated.

iv) Examination of [Electrolytic Method]-2

Measure for Lowering Current Resistance at Electrode Surface—Electrode Materials

To obtain a film within a short time by the electrolytic process system, it is necessary to pass as much current as possible at a voltage of 6 V or below. To pass a greater current, a) it is necessary [to increase the surface area of the electrode when the material is the same], and b) [to lower the current resistance at the electrode surface]. The measure a) is clear to everyone. The measure b) needs the selection of electrode materials that are suitable for the present invention. In other words, it is suitable to select zinc or iron as the electrode material for the positive electrode. Particularly, zinc is a metal that has a large dissolution capacity in phosphoric acid, easily dissolves at a low voltage and makes it possible to pass a large current.

iv) Examination of [Electrolytic Method]-3:

Measure for Lowering Current Resistance of treated article—Adjustment of Oxidation-Reduction Potential (ORP) of Treatment Bath

Because the electrolytic processing according to the invention is processing at a voltage (electrolytic voltage) at which hydrolysis of electrolysis of water does not occur, the flow of the current on the surface of the treated article must be taken into consideration. The observation of the oxidation-reduction potential (ORP) and dissolution of metal materials are known as a Pourbaix diagram. Though the Pourbaix diagram is not based on kinetics, it can be estimated that the oxidation-reduction potential is more advantageously set to the corrosion area (potential) than to the passive area. The metal materials assumed by the invention are iron and steel (inclusive of alloy steel), aluminum, copper, and so forth. These metals are corroded at an oxidation-reduction potential of 300 to 660 mV (hydrogen standard electrode potential). When the oxidation-reduction potential of the treatment bath is adjusted to this range, the current resistance on the surface of the work (treated article) can be lowered and a large current can flow. It is thus effective to adjust the oxidation-reduction potential of the treatment bath to 90 to 450 mV (silver/silver chloride electrode potential) (300 to 660 mV (hydrogen standard electrode potential)).

Preferred examples of the present invention will be hereinafter illustrated. FIG. 1 is a schematic view of a basic phosphating treatment system of the present invention.

The following A or B is used as the treatment bath.

A: The treatment bath uses two kinds of phosphoric acid components constituting the phosphating treatment bath, that is, [H₃PO₄] and [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight]. The invention indicates the dissociation state of phosphoric acid in the treatment bath and uses a treatment bath having the dissolving phosphate ion solution ratio represented by [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight]/[H₃PO₄]+[H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] (weight ratio) within the range of 0.4 to 1. This treatment bath does not contain the composition in which iron is dissolved in phosphoric acid illustrated below.

The treatment bath contains at least 15 g/l of phosphate ions, at least 15 g/l of zinc ions, at least 12.5 g/l of nitrate ions, 0 to 2 g/l of ions of metals selected from the group consisting of nickel, cobalt, copper, chromium and manganese, not greater than 3 g/l of iron ions dissolved from the treated article and the electrode, and not greater than 0.5 g/l of dissolution ions other than those described above.

B: The treatment bath prepared by adding the solution having the [zinc-dissolved phosphate ion solution ratio] of 0.4 to 1 to the solution in which iron is dissolved in advance in phosphoric acid to a concentration not greater than 3 g/l.

The treatment bath contains at least 15 g/l of phosphate ions, at least 15 g/l of zinc ions, at least 12.5 g/l of nitrate ions, 0 to 2 g/l of ions of metals selected from the group consisting of nickel, cobalt, copper and manganese, iron ions dissolved from the treated article and the electrode, and not greater than 0.5 g/l of dissolution ions other than those described above.

The electrolytic process is basically executed by conducting anodic electrolysis and then cathodic electrolysis. In other words, after anodic electrolysis is carried out by using the treated article as the positive electrode and zinc or iron as the negative electrode, cathodic electrolysis is preferably carried out by using the treated article as the negative electrode and zinc or iron as the positive electrode. The anodic electrolysis may be omitted depending on the case.

The anodic electrolysis is carried out by using an auxiliary electrode as the negative electrode and the treated article as the positive electrode. The auxiliary electrode generally uses iron. The cathodic electrolysis is generally carried out by using a main electrode (zinc) as the positive electrode and the treated article as the negative electrode. The impressed voltage is preferably 6 V or less in both kinds of electrolysis. The electrolysis process is preferably executed by setting the cathodic electrolysis current density within the range of 1 to 18 A/dm² and forming the phosphate film on the surface of the treated article. Here, the electrolysis process is carried out under the state where no obstacle hindering the flow of the current exists between the positive and negative electrodes, and the phosphate film is formed on the surface of the treated article.

In the invention, the time for the electrolytic process can be 60 seconds or less but can be extended, depending on the condition and purpose of the electrolytic process, without being limited to the former time.

To form the phosphate film by the invention, it is possible to use two or more power sources and electrodes for one treated article, to apply two or more different voltages and currents depending on the position of the same treated article and to form the phosphate film on the surface of the treated article as illustrated in later-appearing Example 12.

Preferably, the pH of the treatment bath is set to the range of 1.5 to 2.5 and the ORP, to 90 to 450 mV (silver/silver chloride electrode potential) (300 to 660 mV (hydrogen standard electrode potential)).

The invention will be explained by Examples and Comparative Examples but is in no way limited thereto.

EXAMPLES 1 TO 6 & COMPARATIVE EXAMPLES 1 TO 3 Examples in Treatment Bath Not Containing Fe Ions

Test pieces used in Examples 1 to 6 and Comparative Examples 1 to 3 are soft steel material (SPCC material: cold rolled steel sheet) having a size of 50 mm×25 mm×1 mm (t). After degreasing, each test piece is immersed in a titanium type colloidal solution and the phosphating process is carried out to form a film. The electrolytic phosphating process is carried out by anodic treatment (7 seconds)→cathodic treatment (23 seconds). The electrolytic processing time was 30 seconds and was shorter than in the prior art.

Table 1 tabulates the condition under which Examples and Comparative Examples are carried out and their results. In other words, Table 1 tabulates the composition of the phosphating treatment bath, the electrolytic condition, the pH (hydrogen ion concentration) of the treatment bath, ORP (oxidation-reduction potential), the temperature, the total acidity (index representing, by pt. milliliters of a 0.1N caustic soda solution required for the neutralization titration of 10 ml of the treatment bath by the 0.1N caustic soda till coloration to red by using phenolphthalein as an indicator) and the formation condition of the coating. The treatment bath of Table 1 is not a solution which is allowed to intentionally contain Fe ions.

Incidentally, the electrolytic process is a system in which the voltage is elevated to a predetermined voltage and the current is caused to flow between the electrode and the treated article. The electrode materials used for the electrolytic process are tabulated in Table 2 and are zinc and iron.

The electrolytic voltage in the invention is 6 V or less and the maximum voltage in Examples 1 to 6 and Comparative Examples 1 to 3 is 3 V. The impressed voltage of 3 V is a voltage that suppresses decomposition of water. TABLE 1 Comparative Comparative Example 1 Example 1 Example 2 Example 2 Example 3 13 23 15 14 6 treatment dissociation [phosphoric acid 0 0.5 1 0 0.67 bath index of from H₃PO₄(100) + composition phosphoric Zn(25)]/[phosphoric acid acid from H₃PO₄(100) + Zn(25)] + [phosphoric acid from H₃PO₄] phosphoric total phosphoric 30 30 30 30 30 acid acid concentration: g/l concentration phosphoric acid 0 15 30 0 20 concentration from H₃PO₄(100) + Zn(25): g/l phosphoric acid 30 15 0 30 10 concentration from H₃PO₄: g/l zinc total zinc 15 15 15 20 20 concentration concentration: g/l Zn concentration from 0 3.75 7.7 0 5 H₃PO₄(100) + Zn(25): g/l Zn concentration from 15 11.25 7.3 20 15 Zn nitrate: g/l nitrate ion total NO₃— 31 23.5 16 41 31 concentration concentration: g/l NO₃— concentration 30 22.5 15 40 30 from Zn nitrate: g/l NO₃— concentration 1 1 1 1 1 from Ni nitrate: g/l nickel Ni concentration 0.5 0.5 0.5 0.5 0.5 concentration from Ni nitrate: g/l electrolytic anodic voltage 2 2 2 2 2 condition electrolysis current: A/work −0.2 −0.2 0.6 −0.2 −0.2 time rise for rise for rise for rise for rise for 4 sec and 4 sec and 4 sec and 4 sec and 4 sec and holding holding holding holding holding for 3 sec for 3 sec for 3 sec for 3 sec for 3 sec Zn: voltage 3 3 3 3 3 cathodic electrolysis current: A/work 1.3 1.7 1.1 1.4 1.8 time rise for rise for rise for rise for rise for 5 sec and 5 sec and 5 sec and 5 sec and 5 sec and holding holding holding holding holding for 18 sec for 18 sec for 18 sec for 18 sec for 18 sec current: A/dm² 5.2 6.8 4.4 5.6 7.2 iron: voltage 2 2 2 2 2 cathodic electrolysis current: A/work −0.2 −0.2 −0.2 −0.2 −0.2 time rise for rise for rise for rise for rise for 5 sec and 5 sec and 5 sec and 5 sec and 5 sec and holding holding holding holding holding for 18 sec for 18 sec for 18 sec for 18 sec for 18 sec treatment PH 1.48 1.59 1.97 1.51 1.67 bath ORP: mv (silver/ 425 265 262 376 450 condition silver chloride electrode potential) temperature: ° C. 30-35 30-35 30-35 30-35 30-35 total acidity: pt. 70 78 84 72 76 formed film coating ratio: % 70 100 100 100 100 film thickness: μm 1.2 5 8.8 1.8 4.5 Comparative Example 4 Example 3 Example 5 Example 6 16 19 7 17 treatment dissociation [phosphoric acid 1 0 0.67 1 bath index of from H₃PO₄(100) + composition phosphoric Zn(25)]/[phosphoric acid acid from H₃PO₄(100) + Zn(25)] + [phosphoric acid from H₃PO₄] phosphoric total phosphoric 30 30 30 30 acid acid concentration: g/l concentration phosphoric acid 30 0 20 30 concentration from H₃PO₄(100) + Zn(25): g/l phosphoric acid 0 30 10 0 concentration from H₃PO₄: g/l zinc total zinc 20 30 30 30 concentration concentration: g/l Zn concentration from 7.7 0 5 7.7 H₃PO₄(100) + Zn(25): g/l Zn concentration from 12.3 30 25 22.3 Zn nitrate: g/l nitrate ion total NO₃— 26 61 51 46 concentration concentration: g/l NO₃— concentration 25 60 50 45 from Zn nitrate: g/l NO₃— concentration 1 1 1 1 from Ni nitrate: g/l nickel Ni concentration 0.5 0.5 0.5 0.5 concentration from Ni nitrate: g/l electrolytic anodic voltage 2 2 2 2 condition electrolysis current: A/work 0.6 −0.2 −0.2 −0.2 time rise for rise for rise for rise for 4 sec and 4 sec and 4 sec and 4 sec and holding holding holding holding for 3 sec for 3 sec for 3 sec for 3 sec Zn: voltage 3 3 3 3 cathodic electrolysis current: A/work 1.3 3.2 1.7 2 time rise for rise for rise for rise for 5 sec and 5 sec and 5 sec and 5 sec and holding holding holding holding for 18 sec for 18 sec for 18 sec for 18 sec current: A/dm² 5.2 12.8 6.8 8 iron: voltage 2 2 2 2 cathodic electrolysis current: A/work −0.2 −0.2 −0.2 −0.2 time rise for rise for rise for rise for 5 sec and 5 sec and 5 sec and 5 sec and holding holding holding holding for 18 sec for 18 sec for 18 sec for 18 sec treatment PH 1.97 1.39 1.66 2.02 bath ORP: mv (silver/ 233 317 425 211 condition silver chloride electrode potential) temperature: ° C. 30-35 30-35 30-35 30-35 total acidity: pt. 80 95 84 80 formed film coating ratio: % 100 80 100 100 film thickness: μm 11.8 5 5.4 12

Examples 1 and 2 and Comparative Example 1 use the same chemical components treatment bath. The difference between the Examples and the Comparative Example resides in the difference of the degree of dissociation of phosphoric acid. Comparative Example 2 and Examples 3 and 4 also use the same chemical components treatment bath composition, too, and the difference resides in the degree of dissociation of the phosphoric acid. This also holds true of Comparative Example 3 and Examples 5 and 6.

FIG. 2 shows the formation state of the film. The drawing shows that the film is reliably formed within a short time of 30 seconds in an Example though the phosphate film is not always formed reliably in a Comparative Example.

FIG. 3 shows the film formation condition in terms of the film thickness. The film thickness is measured by using an electromagnetic film thickness meter LE-300J, produced by K. K. Ketto Kagaku Kenkyusho. The film formation is excellent in the Examples though it is poor in the Comparative Example. The film thickness is 5 μm in Comparative Example 3 and this film thickness is the thickness of the portion at which the film is formed. In Comparative Example 3, the film is formed on only 80% of the surface.

FIG. 4 comparatively shows the current that flows when 3 volts are applied between the zinc electrode and the test piece during cathodic electrolysis. The diagram shows that the current rises with the increase of the ratio of zinc to phosphoric acid. The current density is maximal in Comparative Example 3 but the film formation is not reliable. To reliably conduct the film formation, it is more important to control the dissociation state of phosphoric acid in the solution than the current density.

The result given above indicates that the process and the treatment bath according to the invention are effective for forming the film within a short period of 30 seconds.

EXAMPLES 7 AND 8 AND COMPARATIVE EXAMPLE 4

Examples 7 and 8 and Comparative Example 4 are cases where actual components (material: SUJ2: high carbon chromium bearing steel: C:1%, Cr:1.45%) are used. The component is the one that is used for a brake system of a car, and is shaped into a gear-like structure by a cold forging press after lubrication treatment. This shaping process is to change the shape shown in FIG. 5 to the shape shown in FIG. 6. Therefore, the component used in Examples 7 and 8 and Comparative Example 4 is the component shown in FIG. 5.

Comparative Example 4 represents the treatment process according to the prior art and is a non-electrolytic system. Examples 7 and 8 are the treatment under the same condition with the exception of the electrolytic treatment time. Table 2 shows the outline for executing this treatment. TABLE 2 Comparative Example 4 Non-electrolytic Example 7 Example 8 treatment treatment bath dissociation index [phosphoric acid from 0.43 0.43 Palbond 3684X composition of phosphoric acid H₃PO₄(100) + (formation chemical Zn(25)]/ [phosphoric acid from produced by Nippon H₃PO₄(100) + Parkerizing Co., Ltd) Zn(25)] + [phosphoric acid from H₃PO₄) phosphoric acid total phosphoric acid 42 42 concentration concentration: g/l phosphoric acid concentration from 18 18 H₃PO₄(100) + Zn(25): g/l phosphoric acid concentration from 24 24 H₃PO₄: g/l zinc total zinc concentration: g/l 22.5 22.5 concentration Zn concentration from 4.5 4.5 H₃PO₄(100) + Zn(25): g/l Zn concentration from 18 18 Zn nitrate: g/l nitrate ion total NO₃— 38 38 concentration concentration: g/l NO₃— concentration 36 36 from Zn nitrate: g/l NO₃— concentration 2 2 from Ni nitrate: g/l nickel Ni concentration from 1 1 concentration Ni nitrate: g/l electrolytic anodic voltage 1.5 1.5 no electrolysis, condition electrolysis treatment time: 600 sec current: A/work 0.2 0.2 time rise for 3 sec rise for 4 sec and holding and holding for 2 sec for 6 sec Zn: cathodic voltage 3 3 electrolysis current: A/work 1.4 1.4 time rise for 3 sec rise for 4 sec and holding and holding for 7 sec for 16 sec current: A/dm² 5.2 5.2 iron: cathodic voltage 2 2 electrolysis current: A/work −0.2 −0.2 time rise for 3 sec rise for 4 sec and holding and holding for 7 sec for 16 sec treatment bath PH 1.99 2 — condition ORF: mv (silver/silver chloride 400 355 — electrode potential) temperature: ° C. 32 32 80 total acidity: pt. 82 82 43 promoter concentration — — 3 formed film cover ratio: % 100 100 100 lubrication effective component: 4.5 4.4 1.5 deposition weight: g/m² phosphate film: deposition 6.7 10 4 weight: g/m² press cold forging press: machining 67 67 68 machinability load mean value: ton

FIGS. 7 to 9 show the appearances of the phosphate films formed. FIG. 7 shows Example 7, FIG. 8 shows Example 8 and FIG. 9 shows Comparative example 4.

The treated articles are treated through the steps of degreasing—surface adjustment→phosphating treatment→lubrication treatment (immersion in sodium stearate solution at 80° C. for 3 min) (surface adjustment is omitted in Comparative Example 4).

The difference between Examples and Comparative Examples is remarkable in the actual treatment. The treatment time is 10 minutes in the Comparative Example but is 15 seconds and 30 seconds in the Examples. The appearances of the formed films are also remarkably different between the Examples and the Comparative Examples. The films of the Examples are compact and the zinc phosphate crystal is uniformly formed on the surface of the treated article. Therefore, the foundation steel is not directly exposed. In contrast, the phosphate film of Comparative Example (prior art) is composed of coarse zinc phosphate crystals. Consequently, whereas the iron surface is reliably covered with the film in the method of the present invention, it can be observed, through an SEM, that the foundation steel is exposed by the film in the method of the prior art.

The lubrication (treatment) film acts on the zinc phosphate film and is formed. Therefore, it is important as to whether the steel surface is reliably covered with the film having the zinc phosphate crystal or is not covered reliably. When the surface is reliably covered with the zinc phosphate crystal, the lubrication treatment bath (Na stearate) does not come into direct contact with the steel surface. When the film is incomplete, however, the lubrication treatment bath (Na stearate) comes into direct contact with the steel surface. This impedes the operation of the lubrication components and forms an incomplete lubrication film. In other words, when the steel components (iron ions, etc) dissolve in the lubrication treatment bath, the iron ions coagulate the lubrication film (a uniform dispersion of soap) and impedes its uniform formation. In other words, a film containing the coagulation components is formed. The lubrication film (soap) should be uniformly dispersed, exist in the film form and thus operate effectively. It is therefore important to form a sufficient zinc phosphate film and the present invention accomplishes this object.

The difference between the Examples and the Comparative Example reflecting the condition described above can be confirmed from the appearance of the formed film. In the lubrication effective component formed uniformly as the lubrication film component shown in Table 2, the mass ratio of Example/Comparative Example is 3/1 and the Example is greater than, and superior to, Comparative Example. The zinc phosphate deposition amount is different between Example 7 and Example 8 and the lubrication effective component remains substantially at the same level. This is presumably because the surface is reliably covered with the zinc phosphate film as can be confirmed, through an SEM, in Examples 7 and 8.

Incidentally, the lubrication effective component tabulated in Table 2 is measured by immersing the film in the lubrication treatment bath (Na stearate bath) during the formation of the lubrication film, manually removing the solid contents (solid contents such as Na stearate separated from the film) precipitating on the surface and measuring the components forming the uniform and continuous film. The measurement of the film is conducted by immersing the film in isopropyl alcohol boiling at 70° C. for 15 to 20 minutes, measuring the weights of the treated article before and after immersion and converting them to values per unit area (m²). This measurement method of the lubrication film (reaction soap) is different from the prior art method that is applied to film formation by the non-electrolytic method. The difference in the lubrication film formation methods reflects the fact that the non-electrolytic treatment method according to the prior art cannot form a complete zinc phosphate film but that the method of the present invention can form it.

The deposition amount of the zinc phosphate crystal is important but it has been confirmed that the difference in the film formation condition is important, too. No difference occurs in Examples and Comparative Example as to the cold forging press load, but it can be confirmed that the Examples are more advantageous from the difference in the properties of the film.

As a result of X-ray diffraction, it was confirmed that the films of Examples 7 and 8 and Comparative Example 4 contain zinc phosphate crystals, though this is not shown in the drawings.

EXAMPLE 9 AND COMPARATIVE EXAMPLES 5 AND 6

Example 9 and Comparative Examples 5 and 6 represent the case where the degree of dissociation of phosphoric acid is adjusted by dissolving iron in phosphoric acid, without dissolving zinc in phosphoric acid.

Table 3 represents the outline of the Examples. In Examples 9 and Comparative examples 5 and 6, [Zn25% dissolving phosphate ion solution ratios] represented by [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight]/[H₃PO₄]+[H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] (mass ratio) are all zero. [Zn25% dissolving phosphate ion solution ratio]=0 is the same as in Comparative examples 1 to 3. However, it is novel that the degree of dissociation of phosphoric acid is regulated by dissolving iron in phosphoric acid. TABLE 3 Comparative Comparative Example 10 Example 11 Example 5 Example 6 Example 9 24 25 26 27 28 treatment bath dissociation index [phosphoric acid from 0.67 0.67 0 0 0 composition of phosphoric acid H₃PO₄(100) + Zn(25)]/[phosphoric acid from H₃PO₄(100) + Zn(25)] + [phosphoric acid from H₃PO₄] phosphoric acid total phosphoric acid 30 30 30 30 30 concentration concentration: g/l phosphoric acid concentration 20 20 0 0 0 from H₃PO₄(100) + Zn(25): g/l phosphoric acid ion from 10 10 30 15 15 phosphoric acid + iron phosphoric acid concentration from 0 0 0 15 15 H₃PO₄: g/l zinc total zinc concentration: g/l 18 18 20 10 20 concentration Zn concentration from 5 5 0 0 0 H₃PO₄(100) + Zn(25): g/l Zn concentration from 13 13 20 10 20 Zn nitrate: g/l nitrate ion total NO₃— 27 27 41 21 41 concentration concentration: g/l NO₃— 26 26 40 20 40 concentration from Zn nitrate: g/l NO₃— 1 1 1 1 1 concentration from Ni nitrate: g/l nickel Ni concentration from 0.5 0.5 0.5 0.5 0.5 concentration Ni nitrate: g/l Fe Fe ion from phosphoric 1 2 6 3 3 concentration acid + iron electrolytic anodic voltage 2 2 2 2 2 condition electrolysis current: A/work −0.2 −0.2 0.6 0.8 1 time rise for 4 sec rise for 4 sec rise for 4 sec rise for 4 sec rise for 4 sec and holding and holding and holding and holding and holding for 3 sec for 3 sec for 3 sec for 3 sec for 3 sec Zn: cathodic voltage 3 3 3 3 3 electrolysis current: A/work 1.5 1.3 0.9 0.9 1.4 time rise for 5 sec rise for 5 sec rise for 5 sec rise for 5 sec rise for 5 sec and holding and holding and holding and holding and holding for 18 sec for 18 sec for 18 sec for 18 sec for 18 sec current: A/dm² 6 5.2 3.6 3.6 5.6 iron: cathodic voltage 2 2 2 2 2 electrolysis current: A/work −0.2 −0.2 −0.2 −0.2 −0.2 time rise for 5 sec rise for 5 sec rise for 5 sec rise for 5 sec rise for 5 sec and holding and holding and holding and holding and holding for 18 sec for 18 sec for 18 sec for 18 sec for 18 sec treatment bath PH 1.93 1.9 2.22 1.5 1.49 condition ORP: mv (silver/silver 130 114 85 179 184 chloride electrode potential) temperature: ° C. 30-35 30-35 30 30 30 total acidity: pt. 74 75 58 58 64 formed film coating ratio: % 100 100 100 70 100 film thickness: μm 6.2 5.4 5 2 3.6

Example 9 and Comparative Example 5 and 6 use a steel material (SPCC material: cold rolled steel sheet) having a size of 50 mm×25 mm×1 mm (t) as the test piece. After being degreased, each test piece is immersed in a titanium type colloidal solution for the surface adjustment and is then subjected to the phosphating process to form the film. The phosphating process is carried out in a cycle of anodic treatment (7 sec)→cathodic treatment (23 sec). The electrolytic treatment time is 30 seconds and is shorter than the prior art technology.

Comparative Examples 5 and 6 use the treatment bath that does not use the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺), and cannot provide the effect of film formation according to the invention.

Example 9 is the example where the film formation can be carried out in the presence of the Fe ion but this bath generates sludge due to the Fe ion.

EXAMPLES 10 AND 11

In Examples 10 and 11, the adjustment of the degree of dissociation of phosphoric acid is made by using two methods, that is, the method that dissolves iron in phosphoric acid to adjust dissociation and the method that uses [Zn25% dissolving phosphate ion solution ratio represented by [H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight]/[H₃PO₄]+[H₂PO₄ ⁻:100 parts by weight+Zn²⁺:25 parts by weight] (mass ratio). Both examples 10 and 11 can form a film.

The remarkable difference between Examples 1 to 8 that do not use the Fe ion for the dissociation adjustment of phosphoric acid and Examples 9 to 11 that use the Fe ion for dissociation adjustment resides in the oxidation-reduction potential (ORP) of the treatment bath. The ORP is higher than 200 mV (silver-silver chloride electrode potential) in all the former (bath not using Fe) while it is lower than 200 mV in the latter (bath using Fe). This indicates that the dissolved Fe ion greatly affects the ORP of the solution (lowers the ORP). To adjust the ORP of the treatment bath, therefore, it is possible to use a [solution adjusting the degree of dissociation of phosphoric acid by dissolving a small amount of iron in phosphoric acid].

EXAMPLE 12

The component B shown in FIG. 10 is a tubular component having a cylindrical hollow portion. When the electrolytic treatment is conducted for such a component, a flow of the current to the pipe inner diameter portion drops and film formation at that portion is restricted. In Example 12, an auxiliary electrode different from the main electrode is inserted into the hollow portion and an electrolytic treatment system separate from the main electrolytic treatment circuit is formed by using the auxiliary power source different from the main power source to conduct the electrolytic treatment. The solid matter of the metal ions (metal material) described in claims 1 and 7 can be used for the electrode material of the auxiliary electrode.

The film can be reliably formed on the pipe inner diameter portion by using such an auxiliary electrode.

The present invention can shorten the phosphating treatment time of a component, as a treated article, and can improve the performance of the resulting film. The reduction of the treatment time makes it possible to produce a dedicated treatment machine and to reduce its size, and to reduce the installation area for the equipment and to convert it to an in-line (transfer line) equipment. The obtained uniform zinc phosphate crystals are particularly effective for cold-forging machining. 

1. An electrolytic phosphating process using a treatment bath that is formed of a phosphate ion solution (H₂PO₄ ⁻+Zn²⁺), made by dissolving zinc in phosphoric acid, contains phosphoric acid (H₃PO₄), phosphate ions, zinc ions and nitrate ions, may contain at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions, and further contains 0.5 g/l or below of metal ions other than said film forming components, said process comprising the step of conducting an electrolytic treatment by applying a voltage between a metal as a positive electrode and a treated article as a negative electrode and forming a phosphate film on the surface of the treated article.
 2. An electrolytic phosphating process according to claim 1, wherein said electrolytic treatment is carried out by using a treatment bath the pH of which is adjusted to 1.5 to 2.5 and the ORP (oxidation-reduction potential) of which is kept at 90 to 450 mV (silver/silver chloride electrode potential).
 3. An electrolytic phosphating process according to claim 1, which contains at least 15 g/l of phosphoric acid and phosphate ions, at least 15 g/l of zinc ions, at least 12.5 g/l of nitrate ions, and 0 to 3 g/l of at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions.
 4. An electrolytic phosphating process according to claim 1, wherein said positive electrode is selected from zinc or iron.
 5. An electrolytic phosphating process according to any of claim 1, wherein, after anodic electrolysis is carried out by using said treated article as the positive electrode and zinc or iron as the negative electrode, cathodic electrolysis is carried out by using said treated article as the negative electrode and zinc or iron as the positive electrode.
 6. An electrolytic phosphating process according to claim 1, wherein a voltage of not greater than 6 V is applied by connection to a D.C. power source.
 7. An electrolytic phosphating process according to claim 1, wherein said phosphate ion solution (H₂PO₄ ⁻+Zn²⁺) dissolving zinc in phosphoric acid is a solution prepared by dissolving 8 parts by mass to a maximum dissolution concentration of zinc in 100 parts by mass of phosphoric acid.
 8. An electrolytic phosphating process according to claim 1, wherein said phosphate ion solution (H₂PO₄ ⁻+Zn²⁺), made by dissolving zinc in phosphoric acid, is a solution prepared by dissolving 15 to 25 parts by mass of zinc in 100 parts by mass of phosphoric acid.
 9. An electrolytic phosphating process according to claim 1, wherein said phosphate ion solution (H₂PO₄ ⁻+Zn²⁺), made by dissolving zinc in phosphoric acid, is a solution prepared by dissolving zinc oxide, zinc hydroxide or metallic zinc in a phosphate ion solution.
 10. An electrolytic phosphating process according to claim 1, wherein a ratio of the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺) and phosphoric acid (H₃PO₄) in said electrolytic treatment bath has a ratio of 0.4 to 1 represented by the relation [phosphate ion solution in which zinc is dissolved (H₂PO₄ ⁻+Zn²⁺)]/[phosphate ion solution in which zinc is dissolved (H₂PO₄ ⁻+Zn²⁺)+phosphoric acid (H₃PO₄)].
 11. An electrolytic phosphating process according to claim 1, wherein said electrolytic treatment bath contains the phosphate ion solution (H₂PO₄ ⁻+Zn²⁺) dissolving zinc in phosphoric acid, phosphoric acid (H₃PO₄) and zinc nitrate, and may contain a metal nitrate constituted by at least one kind of nitrate selected from the group of nickel nitrate, cobalt nitrate, copper nitrate and manganese nitrate.
 12. An electrolytic phosphating process according to claim 1, wherein the electrolytic treatment is carried out at a cathodic electrolysis current density of 1 to 18 A/dm² to form a phosphate film on the surface of the treated article.
 13. An electrolytic phosphating process according to claim 1, wherein the electrolytic treatment is carried out under a condition free of any obstacles, that may impede the flow of a current between the positive electrode and the negative electrode, to form a phosphate film on the surface of the treated article.
 14. An electrolytic phosphating process according to claim 1, wherein a zinc phosphate film is formed by setting the electrolytic treatment time to 60 seconds or less.
 15. An electrolytic phosphating process according to claim 1, wherein two or more kinds of voltages and currents are applied to one treated article, depending on positions thereof, by using two or more power sources and electrodes. 